This invention relates generally to machines which operate in cryogenic superconductive states, and more particularly, to a coolant feed system for a superconductive electric machine, illustratively a turbo-generator, in which a cryogenic coolant in liquid and gaseous phases is delivered to the machine by stationary coolant feed lines, connected to external coolant sources.
One coolant replenishing system which allows flood or bath cooling of a superconducting winding in an electric machine, particularly a turbo-generator is described in German Offenlegungsschrift No. 29 23 496. The system described therein contains an antechamber which is disposed near the axis of rotation of the machine, and rotates therewith. A further cooling device, which contains a rotating mixing chamber in which coolant in liquid and gaseous phase states is contained, is disclosed in German Patentschrift No. 28 30 887 C3. In operation, the rotation of the mixing chamber causes the liquid and gaseous phases of the coolant to separate from each other by centrifugal forces. Thus, gaseous coolant settles in the portions of the mixing chamber which are near the axis of rotation, and the liquid coolant, which is used for cooling the field winding of the machine, remains in the portion of the mixing chamber away from the axis.
The liquid coolant is distributed throughout the electric machine by a self-pumping effect utilizing thermo-siphon loops. Such thermo-siphon loops operate using the principle that the liquid and the gaseous phases of the coolant are characterized by different specific densities. A coolant distribution system is provided at the outer circumference of the field winding. The coolant distribution system is connected by a plurality of cooling canals which conduct the coolant through the field winding and to the outer portion of the mixing chamber.
In an operating machine, all cooling canals in the field winding are completely flooded and filled with liquid helium from the coolant distribution system at the outer circumference of the winding. As the coolant absorbs heat from the field winding and the external environment, its density decreases. This decrease in density causes the coolant to flow in the cooling canals towards the mixing chamber. Simultaneously, colder and therefore denser coolant flows radially outward by means of coolant connecting lines into the coolant distribution system, and subsequently to the cooling canals. A pressure gradient develops along the cooling canals through the winding as a result of heat absorption. This produces convection flow in the system in the form of thermo-siphon loops (see: "Cryogenics", July, 1977, pages 429-433; and DE-OS No. 25 30 100). The rate of the circulating flow is increased as larger amounts of heat are absorbed by the coolant, thereby producing a fail-safe cooling operation.
The gaseous coolant which is located near the axis of rotation in the mixing chamber is advantageously utilized to produce a counter flow for cooling connecting elements of the body of the rotor which carries the field winding. Such cooling substantially reduces the amount of heat which is introduced into the machine from the environment. In the course of such cooling, the gaseous coolant absorbs sufficient heat to be raised from a few degrees K to approximately room temperature, and becomes correspondingly less dense. Since the warming-up of the gaseous coolant takes place at a long radius from the axis of rotation, but the cold gaseous coolant enters the loop in the vicinity of the axis of rotation, a pumping effect is achieved. If the output pressure of the gaseous coolant from the mixing chamber is maintained at a constant pressure, illustratively 1.1 bar, the resulting pump would produce an under pressure in the mixing chamber of illustratively between 0.3 and 0.6 bar. As a result of the thermodynamic characteristics of helium, the reduction in pressure causes a drop of about 1 degree K, and, therefore a higher current-carrying capacity in the superconducting winding.
In such a cooling arrangement, coolant must be replenished at low pressure and low temperature. A refrigerator plant must therefore be provided which supplies undercooled helium at low pressure. In addition to costing more than a plant which operates at normal pressure, difficulties are encountered in the maintaining of undercooled helium. Moreover, the extremely cold machine parts draw warm gases from the environment, as a result of the underpressure, thereby creating difficulties in sealing the system.
As a result of these problems, pressure reduction stages which rotate with the winding have been provided so as to permit the refrigeration plant and the coolant feed lines to be operated at an optimum pressure of approximately 1.2 bar. One such pressure reduction stage which is provided in a system for replenishing helium which is conducted from a helium supply tank to a helium bath in the rotor of a superconducting generator is described in the above-mentioned German patent application P No. 29 23 496.6. In this system, a rotating antechamber which is located near the axis of rotation of the generator is provided in a coolant feed system. Helium is supplied to the antechamber from an external supply by means of a stationary coolant feed line. Since this helium is in liquid and gaseous phases, the gaseous coolant settles near the axis of rotation when the rotor rotates. The liquid coolant is separated from the gaseous coolant, and occupies a region in the antechamber which is radially away from the axis of rotation. Thus, the antechamber operates as a phase separator.
In this arrangement, the pressure differential between the coolant pressure in the antechamber and the underpressure in the rotating helium bath for the field winding is achieved by a relatively warm coolant column in a feed line between the supply chamber and the helium bath. The coolant column is maintained at a pressure which equals the colder coolant column of the helium bath. The level of the liquid-gas phase boundary in the rotating helium bath for the field winding is determined by the radius of the liquid-gas phase boundary in the antechamber. Accordingly, if the proportion of liquid coolant in the helium bath decreases as a result of losses, then the pressure of the rotating coolant column in the bath must decrease correspondingly in order that coolant can flow from the antechamber into the bath. The liquid coolant flowing out of the antechamber must then be replaced with coolant which is fed in from the outside. The level of the liquid-gas phase boundary in the phase separator must therefore be controlled by level controllers in the coolant replenishing system. Such level controllers, which may be temperature monitoring sensors which operate in conjunction with control valves, are disadvantageously large, expensive, and unreliable.
It is, therefore, an object of this invention to provide a coolant replenishing system for an antechamber in a superconducting electric machine, wherein the supply of coolant which is provided from an external source is controlled as a function of coolant loss.